The distinctive belt of asteroids between Mars and Jupiter may have been shaped during a game of planetary pinball almost 4 billion years ago. A new study suggests that the migration of the mighty gas giant planets tugged some asteroids into a ringed formation, and sent others spinning off. Jupiter, Saturn, Uranus, and Neptune, are thought to have been born close together before gravitational interactions with numerous pieces of rocky debris changed their trajectories. Their movement then caused the rocky debris to scatter like bowling pins, potentially explaining what battered the Earth, Moon, and Mars with so many craters some 3.8 billion years ago [New Scientist].

Astronomers have long wondered about the uneven distribution of debris within the asteroid belt, which has zones where there are far fewer asteroids than expected…. Some of those gaps, called Kirkwood gaps, are in zones where Jupiter or Saturn’s gravitational influence destabilises the asteroids so much that they are ejected from the belt, but many are in areas that are currently stable [Cosmos]. Researchers decided to test the theory that planetary migrations caused the gaps in the solar system‘s early days.

Planetary scientist David Minton and his colleagues designed a computer model of the asteroid belt under the influence of the outer “gas giant” planets, allowing them to test the distribution that would result from changes in the planets’ orbits over time. A simulation wherein the orbits remained static, Minton says, did not agree with observational evidence. “There were places,” he says, “where there should have been a lot more asteroids than we saw” [Scientific American]. But as they describe in a paper in Nature, a simulation using migrating planets showed the planets interacting with the rocky debris in specific areas, sending asteroids hurtling out of the belt and carving virtual furrows that closely match the real thing.

The study also suggests that Jupiter has drifted inward toward the sun, whereas Saturn and the other gas giants drifted outward. However, Minton says the findings can’t predict “exactly when the migration took place” or the precise paths of the planets. On the other hand, he says, “we hoped to see an unambiguous record of the migration, and that’s what we think we’ve found” [ScienceNOW Daily News].